US2499759A - Location of faults in electrical transmission systems - Google Patents

Description

March 7, 1950 R. A. KEMPF 7 2,499,759

LOCATION OF FAULTS IN ELECTRICAL TRANSMISSION SYSTEMS Filed July 31, 1947 FIG. /A

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IN V E N 70/? By R. A KEMP/'- ONE 510: OPPOSITE 5/05 sr/uoau? or FAULT or nun FAULT ATTOR EV Patented Mar. 7, 1950 AtQJSQ LOCATION OF FAULTS IN ELECTRICAL TRANSMISSION SYSTEMS Raymond A. Kempf, Baltimore, Md, assignor to Bell. Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 31, 1947, Serial No. 765,087 6 Claims. (Cl. 175-183) This relates in general to the location of faults in an electrical transmission system. More particularly, it relates to the location of high-voltage faults in electrical transmission lines.

In certain types of coaxial cable systems, breakdown at high voltage is frequently caused by the presence of mechanical defects or metallic inclusions which may not be located by the usual prior art methods. Moreover, in accordance with certain other prior art systems, the faulty point is located only in terms of length ratios whereby the region, but not the exact point of fault, is known. If the length is not precisely known, the cable system must be opened over a considerable distance for visual inspection and listening tests, which procedure is time consuming, particularly from the standpoint that manual rebuilding of the inspected. section frequently causes mechanical and electrical degradation of the cable system.

It is therefore the primary object of this invention to provide techniques and apparatus for locating the precise physical points of highvoltage faults in electrical transmission systems.

A particular feature of the present invention is its adaptability for the large-scale factory inspection of certain types of cable for high-voltage faults. The invention is particularly useful for the location of high-voltage faults in disk-insulated longitudinal seam coaxial cable, whether in single or twisted multiple strands, and also in other types of cable circuits such as lightning protected cable having a sheath-to-corrugatedcopper path.

In accordance with the present invention, the test cable is fed from one to another of a pair of reels, condensers being slidably connected between the central conductor and sheath through slip rings at each end of the test interval. A high enough direct-current voltage is applied to one end of the circuit toproduce an electromagnetic disturbance which may take the form of a flash-over at the fault. The intermittent discharges of one or both condensers through a flash-over point at the fault produce 1. R. drops along the sheath which can be measured and phase-compared in an indicator, such as a cathode-ray oscilloscope, which is electrically coupled to the cable through a rolling or sliding pick-up device. The exact location of a fault can then be effected by a determination of the point at which phase reversal occurs on the indicator as a current detecting means or, what is more commonly known in the engineering vernacular as a pick-up device is moved. over the test inter val. Several alternative forms of pickup and indicating circuits are disclosed.

The invention will be better understood and other objects and features thereof will be apparent from a study of the drawings and the detailed description set forth hereinafter.

In the drawings:

Fig. 1A shows a high-voltage fault locating system in accordance with the present invention mounted for examination of cable passing between two reels;

Figs. 1 (B-E) show modified forms and positions of the pick-up device I9 of Fig. 1A;

Fig. 1F shows a further modification of the system of Fig. 1A in which a galvanometer is substituted for the cathode-ray tube indicator 25;

Fig. 2 is a schematic diagram of the equivalent electrical circuit indicating the principle of operation of the system of Fig. 1A; and

Figs. 3 (A), 3 (B) and 3 (C) show various types of figures which appear as the screen of the cathode-ray tube indicator 25 during operation of the system of Fig. 1A.

Referring to Fig. 1A of the drawings, one of the preferred embodiments of the invention will now be described in detail.

The test cable I may be a longitudinal seam coaxial cable of the type and size conventionally used for telephone communication, comprising a cylindrical outer conductor 2 and an inner axial conductor 3 which are maintained at a uniform separation by means of thin disks of dielectric material such as hard rubber or polyethylene disposed at regular intervals along the interior of the cable. ihe dielectric medium is thus almost wholly gaseous.

Assume the presence in the test cable 1 of one or more mechanical defects or metallic inclusions 4 which tends to increase the potential gradient between the outer conductor 2 and the inner conductor 3 at that particular point, making it more susceptible to high-voltage breakdown.

The cable 5 is mounted on a pair of reels 5 and 6 which are spaced apart by a convenient distance X, a few yards long. The reels 5 and 6 are so disposed as to be rotatable in a clockwise direction, whereby the cable I may be progressively fed at any desired rate from the reel 5 to the reel 6, moving from left to right across the space interval X.

One terminal of the sheath 2 of the test cable section i is connected to the slip ring 5a of the reel 5, and the other terminal of the sheath 2 is connected to the slip ring 6a of the reel 6. Likewise, the terminals of the inner cable con- 3 ductor 3 are respectively connected to the slip rings 52) and 6b of the reels 5 and 6.

A source of direct current potential H which may comprise, for example, a kenatron tube or other high voltage source, is adapted to be connected between the inner conductor 3 and the sheath 2 of the cable I at the right-hand reel 6. The connecting circuit includes the sliding contacts 1 and 8 riding on the slip rings 6b and 6a thereby respectively making contact with the inner conducting cable 3 and the cable sheath 2; the condenser ii the regulating resistance 12, and the interconnecting switches l3 and I4.

Switches 13 and I4 are so adapted that the source H in parallel with the condenser i is connected between the inner and outer cable conductors 3 and 2 when switch 13 is positioned on contact He and the switch 14 is positioned on the contact Mb.

Alternatively, the source H is adapted to be independently connected to charge up the condenser IO when the switch I4 is in position I41: and the switch [3 is in position I37). In accordance with a third arrangement the switch 13 may be positioned so that the condenser I 0 is disconnected from the circuit and the source H is connected directly between the sheath 2 and the inner conductor 3 of the cable I through the contact I41) of the switch I4.

At the left-hand reel 5, a condenser I1 is provided for connection between contacts l and I6, and hence between the sheath 2 and the inner conductor 3 of the cable I. The switch 18 provides for alternatively connecting condenser I! in and out of the circuit.

The cable I is threaded through a pick-up device |9 which is coupled to a detecting device 20. The pick-up device I9 comprises an inductive strap 23 mounted on the rolling contactors 2| and 22 which are adapted to move over the surface of the cable sheath and make electrical contact therewith.

The detecting device may comprise a cathode-ray oscilloscope having a high-persista-nce screen and provided with vertical deflecting plates 26 and horizontal deflecting plates 21. The vertical deflecting plates 26 are connected through the delay circuit 30 to the coil 24 which is inductively coupled to the contacting strap 23.

The horizontal deflecting plates 21 are connected across the output of the sweep generator 28, which is a conventional type of saw-tooth voltage generator well known in the art. The sweep generator 28 is triggered to synchronize its operation with recurrence of fault-generated vibrations by connection to the cable output through a circuit which includes the condenser 29 in series with the inductive coupling coil 24.

Assume the system of Fig. 1 is to be operated for the purpose of examining a length of cable I for high voltage defects of the type described.

The cable I is mounted on the reels 5 and 6 in the manner described so that upon rotation of the reels 5 and B in a clockwise direction the cable is progressively threaded through the rolling contactors 2| and 22. In the circuit associated with the right-hand reel 5 the switch I4 is positioned on contact Mb; and the switch i3 is positioned on contact 13a, thereby connecting the potential source II and the condenser if! in parallel between the central conductor 3 and the outer conducting sheath 2. In the circuit associated with the left-hand reel 6, the switch 18 is closed, connecting the condenser l1 between the central conductor 3 and the sheath 2.

Assume the presence in the test interval X of a mechanical defect or metallic inclusion 4 which tends to increase the potential gradient between the outer and inner conductors at a particular point, making it more susceptible to high voltage breakdown. If the potential of the source I! is adjusted to a sufiiciently high value, a voltage breakdown occurs at the fault 4 which takes the form of a series of periodic arc discharges between the inner and outer conductors, whose rate is dependent on the constants of the R. C. circuit comprising the cable conductors 2 and 3, the condensers l0 and IT, and the rheostat l2, which may be adjusted to secure the desired timing. The aforesaid discharges initiate trains of waves which travel along the cable conductors in both directions from the fault 4.

When discharge takes place, the condition shown in the equivalent circuit of Fig. 2 will obtain. Condensers [Band I! periodically discharge through the flash-over or breakdown at 4 each time it occurs, and each develops a measurable IR drop along the outer conductor of the unit, where I is the current in the coaxial conductors and R is the surface transfer impedance of the outer conductor. Inasmuch as the condensers discharge from opposite ends of the unit, the individual IR drops developed are in series opposition, with the fault forming the junction of common polarity.

It has been observed experimentally that it is important to keep the impedance of the exterior circuit between the ends of the coaxial cable I relatively high as compared to the impedance of the strap 23. This external circuit is shown on the equivalent circuit of Fig. 2 by the dashed line. The current Is in the external circuit is set up whenever there exists an inequality in voltage drops along the outer conductors, as a result of the position of the point of breakdown relative to the reels. The direction of flow of I3 has been found to depend on the position of the fault with respect to the reels; that is, the direction reverses as the fault is re-reeled from one reel to the other. When the fault is between the reels the current I3 flows in proportion to the difference in the two voltages along the outer surface of the outer conductor produced by the currents I1 and I2 acting through the surface transfer impedance of the outer conductor. Thus, I3 reaches a null when the fault is exactly centered between the reels. It has been found experimentally that a pick-up coil or loop placed in the vicinity of the cable appears to pick up a voltage which is induced by the current I3. Obviously, the induced voltage does not change polarity when the coil is moved past the fault, because I3 flows around the circuit including the tertiary impedance indicated by the dashed line on the drawing. It is this fact that explains the need for the low-impedance strap. The strap 23 provides an impedance path much lower than that through the tertiary circuit, and thus the current through the strap will reverse when the fault is passed. The pick-up coil is coupled to the strap as indicated on the drawing.

By rotation of the reels 5 and 6 the test cable I is fed from left to right across the interval X so that it progressively threads through the rolling contactors 2| and 22 of the inductive strap 23. Assuming the presence of a fault 4 in the test interval X, the screen of the oscilloscope 25 will show a deflection which changes in polarity as the strap 23 passes the fault, such as shown in- Figs. 3Aand-3B.

As indicated in Fig. 3C, no: deflection will be obtained when the strap is centered over the fault.-

This method thus facilitates precise physical: location of the faultor point of flash-over at any convenient speed of re-reeling.

In accordance with an alternativeform of operation either one or the other of the condensers ill or ll may be disconnected from the circuit by open-circuiting their respective switches l3 and I8. In this case, the oscilloscope deflection will be obtained only when the pick-up device 19 is located between the operating condenser and the fault 4.

Short circuits may be located by eliminating the condenser at the far end, and connecting switches l3 and 14 so that the condenser Ill may be alternatively charged and discharged through the cable connection. The resultant discharge may be detected by a deflection appearing on the screen of the oscilloscope 25" when the pick-up device is located between the condenser l and the fault 4.

Although it is conceivable that a system in accordance with the present invention might be operated without either of the externally connected condensers ill and H by placing dependence on the charge collected along the distributed capacitance of the cable I for generation of detectable currents I1 and I2, these currents would be small in magnitude and high in frequency with consequent difficulties.

Other devices for picking up the IR drop along the outer conductor or the field set up by the discharge current flowing in the conductors may be substituted for the pick-up u'nit i9 of Fig. 1A, such as shown in Figs. lB-lF. All of these devices depend upon either the current fiow accompanying the breakdown at the fault to set up a magnetic field which may be detected, or upon the potential difference between adjacent sections of the outer conductor to set up detectable current fiow in a secondary or tertiary circult which is attached to the test coaxial cable unit.

The detectable field has been shown to be dependent upon the relative concentricity of the conductors of the coaxials in addition to the shielding properties of the outer conductor. In the case of eccentricity, the resulting external field may over-shadow that normally present in the absence of eccentricity. Since the degree and direction of eccentricity is likely to occur at random along a given coaxial, the signal detected by the pick-up device may vary over wide limits leading to inconclusive or misleading fault location. Use of a strap such as 23 in Fig. 1A eliminates this confusion.

The pick-up device shown in Fig. 1B, which may be substituted for the pick-up unit [9 of Fig. 1A, comprises a conducting strap 32 having contacting probes 32a and 321) which are adapted to be moved along the cable sheath 2. The fault generated signal is picked up inductively through a solenoid 3i having its axis substantially at right angles to the longitudinal axis of the cable I. The solenoid 3| may be connected to any suitable polarity-sensitive indicator such as the cath0de-ray tube indicator 2!] of Fig. 1A.

Another alternative form of the pick-up device H! is shown in Fig. which utilizes a toroid 35 wound on a high permeability core which is threaded through a conducting strap 34 having rolling contactors 34a and 34b which make electrical contact with the cable sheath 2. Instead 6. of the toroid; 3.6: a single-loop of wire 3.8 asshown in: Fig. 1CD): may be inductively coupled to; a. tertiary circuit including the conducting strap 37 and the cable sheath 2.

Another alternative form of the pick-up device #9 is shown in Fig- 1E, in which a capacitative coupling with the cable sheath 2 is substituted for the direct contact coupling between the sheath and conducting strap, such as shown in Figs. lA-l'D. The contactors 4t and 41 comprise shaped metal shoes lined with dielectric material which are adapted to ride over the cable sheath 2- without making direct electrical contact: therewith. The contactors All and 4! are connected to an indicating device such as the cathode-ray tube indicator 20 described with reference to- Fig. 1A above.

Fig. 1F shows a special type of coupling in which the coaxial under test is threaded through a solenoid. The voltage at terminals of the solenoid 45 willbe zero unless the coaxial cable I is wrapped with helically applied steel tapes 42, as in the case of the standard telephone coaxial, or other conductor applied in direct contact with the copper outer sheath 2. The helical steel tapes or other conductors have current set up in them due to potential differences longitudinally along the coaxial sheath 2. Since the tapes or conductors form an elongated solenoid, they will induce current in. another solenoid, such as 45, which has the same or a parallel axis. Under such conditions, the us of the steel or other helical conductors to induce voltage in the pickup coil is in reality a special case of the use of the strap 23' disclosed in Fig. 1A.

The terminals of the coil 45 are coupled to a polarity-sensitive indicating device, which may take. the form of a cathode-ray tube indicator such as Zll described hereinbefore with reference tov Fig. 1A,. or alternatively, a conventional galvanometer such as 46.

It is apparent that the galvanometer 46 could be substituted for the cathode-ray tube indicator 20- in combination with any of the alternative pick-up devices shown in Figs. 1A-1E. Moreover, it will be apparent to those skilled in the art that within the scope of the invention other equivalent elements and combinations of elements can be used in addition to those disclosed; and the test system is applicable for the testing of other cable units than the type described, such as, for example, stranded cables having two or more coaxials, or lightning-protected cable having corrugated copper sheathing, or flexible coaxial cable having a braided outer conductor. In fact, the system of the present invention is adapted for the location of any core to sheath failure in a cable having a multiplicity of conductors such as, for example, a paper insulated voice frequency or carrier frequency telephone cable.

What is claimed is:

1. The method of locating a high voltage conductor-to-sheath fault in a conductively sheathed cable section which comprises storing electrical charge between said conductorand said sheath of sufficient magnitude to cause repeated voltage breakdown at said fault whereby said stored charge is repeatedly discharged through a circuit including said fault and a portion of said cable section adjacent thereto, and detecting variations in said discharge current in the sheath at difierent points along said cable section for determining the physical location of said fault.

-2. The method of locating a high-voltage conductor-to-sheath fault in a conductively sheathed cable section which comprises repeatedly storing electrical charges between conductor and sheath at both ends of the section of sufiicient magnitude to cause repeated voltage breakdown at the fault whereby said stored charges are repeatedly discharged through the fault and the two intervening portions of cable, and detecting the relative phase of the discharge current in the sheath at difierent points along said cable section to determine the point at which said relative phase reverses.

3. A system for locating faults in a transmission line comprising a plurality of conductors which comprises in combination means for producing periodic capacitance discharges between certain of said conductors at a fault in said line whereby current surges are caused to flow from said fault through a circuit including a section of at least one of said conductors adjacent said fault, a sheath current detecting circuit positioned to move along said conductor in a direction substantially parallel to the direction of current flow therein, and a polarity-sensitive current indicating circuit electrically coupled to said detecting circuit to indicate variations in the magnitude and phase of said current at diiferent points on said conductor.

4. A system for locating faults in a transmission line comprising a plurality of conductors which comprises in combination means for producing periodic capacitance discharges between certain of said conductors at a fault in said line whereby current surges are caused to fiow in both directions from said fault along at least one of said conductors, a sheath current detecting circuit positioned to move along said conductor in a direction substantially parallel to the direction of current flow therein, and a polarity-sensitive current indicating circuit electrically coupled to said detecting circuit to indicate variations in the magnitude and phase of said current at different points on said conductor.

5. A system for testing conductively sheathed cable which comprises in combination means to progressively feed lengths of said test cable across a preselected test interval, at least one condenser slidably mounted in circuit relation to said cable at one end of said test interval, a source of direct current potential connected in circuit relation to charge said condenser to a sufliciently high potential to cause arcing at a fault in said cable passing through said interval, a sheath current detecting circuit slidably mounted to move along said cable within said interval, and a polarity-sensitive indicating circuit coupled to said detecting circuit.

6. A system for testing conductively sheathed cable which comprises in combination means to progressively feed lengths of said test cable across a preselected test interval, a pair of condensers each of which is slidably mounted in circuit relation to said cable at a. different end of said interval, a source of direct current potential connected in circuit relation to charge said condensers to a sufiiciently high potential to cause arcing at a fault in said cable passing through said interval, electrical pick-up means slidably mounted to move along said cable within said interval, and polarity-sensitive electrical indicating means electrically coupled to said pick-up means.

RAYMOND A. KEMPF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,745,419 Henneberger Feb. 4, 1930 1,886,682 Hubbard Nov. 8, 1932 2,176,757 Borden Oct. 17, 1939 2,199,846 Borden May 7, 1940 2,321,424 Rohats June 8, 1943 2,460,688 Gambrill et a1. Feb. 1, 1949 FOREIGN PATENTS Number Country Date 306,679 Germany July 9, 1918

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